front disc/rear drum: ONE drum locks up?

Status
Not open for further replies.
Joined
Jan 18, 2013
Messages
41
Location
Iowa
Hi! Got a problem I haven't seen before in my lengthy (joking) 2 year career as a mechanic, nor do I remember being directly addressed in my few classes at community college. I'm looking at a 1994 Pontiac Bonneville, with front disc, rear drums. Err, I don't think it had abs. I can check, if that's important. A coworker pulled it in and did the "brake eval." What he observed was that whereas you normally expect modulated, even, brake application; the lightest tap on the brakes causes the LR drum to seize up instantly, while the other 3 wheels get progressively firmer braking as expected. Naturally, the vehicle stops hard and pulls every time you even lightly apply the brakes. It's not very driveable, and the customer wants it fixed. Once you let up on the brakes, the pressure releases, the shoes return normally, and it operates normally: so probably the spring hardware is functional, and pressure isn't being held by a bad rubber hose either, for example. I just want to be sure about this. My understanding is that there are two types of proportioning valves: one is for level sensors, to send more brake pressure to the rear first if you heavily load down your SUV, for example. Not all cars have this. Another, sends pressure to the drums first, because they have further to travel than the .010" of the front discs, and exists in all front disc/ rear drum systems. Finally, being a 1994 and not a 1964, I expect it has a split system. So I'm speculating... is it possible that there are two proportioning valves, one for each leg, and therefore, if just the one is sticking, it's sending much too much pressure to just the one drum first, before any other wheel gets pressure? And I should sell a proportioning valve? Again: just want to be sure. I've got coworkers presenting ideas like, "master cylinder" "the brake hose by that wheel cylinder" "new wheel cylinders," and well, we could do it all... but I want to be right the first time. Let me know your expert opinion. thanks! -Bernard
 
Joined
Jan 25, 2008
Messages
4,463
Location
Guilford, CT
The shoes were installed backwards on one side. There is a leading shoe that's supposed to be installed towards the front of the car, and a trailing shoe that gets installed towards the back. They may look extremely similar and can be easy to mix up.
 
Joined
Oct 9, 2011
Messages
5,090
Location
USA
About every GM I have had locked a rear wheel occasionally...I got into a once a month ritual of removing the brake dust from the drum and linings and it seemed to work.Until the dust built up again.These were on factory GM brake linings.Never kept the rigs long enough to replace them with aftermarket to see if there was a difference.GM even issued a TSB for S10s I believe a decade or so ago stating that their linings may "grow" when wet (in rain or dampness),and urged owners to come to GM for new shoes (at the customers expense).
 

berninicaco3

Thread starter
Joined
Jan 18, 2013
Messages
41
Location
Iowa
It does have ABS, just checked. Not that ABS can work on rear drums anyway, so likely moot. The wheel cylinder isn't leaking: wouldnt fluid saturated shoes grip less, anyway? Hmm-- will check for a difference side:side. One would think she'd have noticed that whenever new shoes were last installed and put 2 and 2 together, but, maybe not! And my coworker said they did look almost new: maybe a friend put them on, did this, and she comes to us. One never knows the baggage a car may come with. NHGUY: we just had a big snowstorm yesterday, so there is moisture in the air. Should I suggest shoes for her, then? We've already written her up for wheel cylinders, but i'm uncomfortable/uncertain with the diagnosis, hence this post.
 
Last edited:
Joined
Feb 22, 2011
Messages
1,437
Location
Indiana
Originally Posted By: oldhp
Most probably left wheel cylinder leaking causing brake to grab.
This is my first thought...
 
Joined
Jun 15, 2003
Messages
37,849
Location
ME
Read up on "servo brakes", long story short the shoes rotate when they're applied and the rotation jams them on better. The front/rear thing is important but so is having good hardware and springs. The whole shebang comes in a kit, of course. I'd put on some advance auto silver shoes... grippy but not over the top, cheap, and if they "grow" they'll wear down fast. Just kidding but when I get a decade out of shoes on GM stuff I put cheapies on. Have replaced shoes from glazing (old age) as well and figure if cheapies wear faster that will keep it down. If that car has 3-channel ABS it'll lay off the whole rear axle if one wheel locks up. IIRC Delco VI is 3-channel, you can spot it by two bleeders on the master cylinder or look for a pic on google.
 
Joined
Sep 30, 2009
Messages
532
Location
Michigan
Assuming a 98 is similar, the prop valves are in line, just forward of the drivers side rear tire. Make sure one wasnt discarded or put in backwards during a line repair.
 
Joined
Sep 5, 2006
Messages
19,479
Location
Chicago Area
Fluid on brake shoes does indeed cause stickiness. Shoes installed backwards reduces that drum's effectiveness - causing an imbalance side to side. What is needed here [since he did a visual examination] is a pressure tester on the brake lines. Any disparity, and you have to work your way back to the master while checking pressures.
 

berninicaco3

Thread starter
Joined
Jan 18, 2013
Messages
41
Location
Iowa
Since my colleague looked at it before, I got my first view, and it was only the leading shoe of the LR drum that was locking: it was worn down twice as far as any of the other 3 shoes. I did the wheel cylinder, and new shoes, and new hardware... and seems OK now. Odd. Well, fingers crossed that the concern doesn't pop up again. Shoes definitely weren't soaked or anything: just dry brake dust, as is usual.
 
Joined
Dec 13, 2006
Messages
1,128
Location
midwest
A rusty/frozen parking brake cable can cause early locking and dragging on a drum brake system. They can get rusty both at either wheel, and up closer to the pedal. With one shoe being more worn than the other, this could indicate the parking brake cable or hardware inside the drum sticking. I would pull both drums and make sure both sides move equally when the brake is set.
 
Status
Not open for further replies.
Top